Hereinafter, the term “front” refers to a spark discharge side with respect to the direction of an axis of the spark plug; and the term “rear” refers to a side opposite the front side.
A spark plug for an internal combustion engine generally includes a cylindrical metal shell, a cylindrical insulator formed with a through hole and arranged in the metal shell, a center electrode disposed in a front side of the through hole, a metal terminal disposed in a rear side of the through hole and a ground electrode joined at a base end portion thereof to a front end face of the metal shell and bent so as to define a spark discharge gap between a distal end portion of the ground electrode and a front end portion of the center electrode. It is known, as a technique to prevent radio noise caused by engine operation, to provide a resistor between the center electrode and the metal terminal within the through hole of the insulator.
In recent years, there is a demand to increase the discharge voltage of the spark plug for high output performance of the internal combustion engine. However, the increase of the discharge voltage leads to an increase in high-frequency noise that can affect a vehicle electronic control system. It is thus demanded to suppress the occurrence of high-frequency noise during spark discharge of the spark plug.
There has been proposed various techniques to suppress such high-frequency noise. For example, Japanese Laid-Open Patent Publication No. 2011-159475 proposes the arrangement of a cylindrical ferrite body as a noise suppression member around the resistor in the spark plug. Japanese Laid-Open Patent Publication No. H02-284374 proposes the arrangement of a wound wire in the spark plug.